1. Field of the Invention
The present invention relates to a surface testing means, operating by reflection, for linearly moving, tape-like materials, in particular for an information-carrying tape coated with a magnetic recording layer.
2. Description of the Related Art
When tape-like materials of this type are used, they have to meet high requirements with regard to freedom from errors. In particular, linear surface irregularities transverse to the running direction of the tape are undesirable. These form in particular as a result of creasing during their production in a tape coating machine. In this procedure, a tape-like substrate is unwound from a roll, transported in its running direction through the machine by means of positively driven rollers, coated and finally wound onto a second roll under tension.
It is known that transverse creases can form in the outer wound layer, for example as a result of brief changes in the winding speed or in the tension, and are included in the roll during further winding. As a result of the action of the tension of subsequent wound layers, this leads to permanent, linear deformations of the tape surface. These may render the relevant roll useless. Since such creasing cannot be completely avoided with the conventional winding means, there is a need for suitable surface testing means for detecting the creases or the linear deformations of the tape-like material which are caused by creasing.
An appropriate optical testing means is disclosed, for example, in U.S. Pat. No. 5,130,555. According to this, a pattern of linear, parallel stripes is projected onto the surface of a continuously moving tape-like material and the line pattern reflected by the surface is electronically evaluated. A crease is detected if the reflected pattern deviates from the original with regard to linearity. The tape crease can then be eliminated by a suitable tape guide means.
This technical procedure is, however, very expensive because, for example, a complex test system comprising video camera, video processor and subsequent electronic data processing is required.
This is true particularly when a plurality of tape strips of the tape-like material are to be tested simultaneously, for example on a longitudinal slitting machine for magnetic information media. It is also possible that very fine linear irregularities will not be detected, owing to the limited optical resolution of the imaging system and the digitizing of the reflected pattern. U.S. Pat. No. 5,357,335 and DE-C 32 12 438, too, are not completely satisfactory with respect to reliable registration of fine, linear irregularities of the tape surface. These documents merely provide optical evidence of large, local layer defects on a coated tape, having, for example, a defect diameter greater than 0.5 mm.